The present invention relates to the field of electrical devices for detecting fault conditions, and more particularly to devices that sense the presence of electrical arcs in a circuit. Electrical arcing is a localized, high-energy event caused by wire chafing, dust build up and moisture that may result in a catastrophic fire. However, arcs, like many transients, are very short lived and therefore cause little impact on bimetallic elements or the electronics. Consequently, conventional circuit breakers and electronic wire protection methods do not interrupt circuits fast enough to prevent damage or a fire. There are two distinct types of arcs, series arcs and parallel arcs.
In general, a parallel arc starts when current flows between two touching oppositely charged conductors causing metal at the contact point to liquefy and current to jump over the resulting gap. The arc may then dissipate, but has the potential to re-establish itself at a later time. Parallel arcs may also occur intermittently, usually as a result of vibration, without ever turning into a hard short. FIG. 9 shows a typical arc in a 28VDC system where the arcing event occurs over several seconds.
The arc current in a parallel arc does not pass through the load, and only the source current capability and wiring resistance limit the peak arc current. There are several scenarios in which parallel arcs occur. For example, parallel arcs can occur when wire insulation is missing or damaged and a wire connected to an aircraft power bus lightly contacts the airframe ground (or another exposed wire), or when insulation located in a wet or dirty environment is missing or damaged.
The other type of arc is a series arc. A series arc occurs when a gap, or break in the circuit, develops in series with a load that is normally connected to an aircraft power bus and the load current jumps over the gap. Unfortunately, some loads utilize series arcs as part of normal operation, so they cannot be reliably detected. For example, the normal sparking of a motor commutator cannot be distinguished from a series arc. Arc lamps, i.e., strobes and fluorescent lighting are also hard to distinguish from arcs.
Various types of arc detectors have been developed and/or proposed to detect arc currents. Generally, there are two types of detectors. One type responds to the random high frequency noise content of the current waveform generated by an arc. This high frequency noise tends to be attenuated, especially by the presence of filters on some loads, which can be connected to the branch circuit. The other basic type of arc detector responds to the step increase in current occurring as the arc is repetitively and randomly struck. Examples of arc detectors of the latter type are disclosed in U.S. Pat. Nos. 5,224,006 and 5,691,869.
These prior art devices have difficulty distinguishing arc currents from normal inrush currents. Inrush currents, e.g., capacitive inrush currents, lamp inrush currents, or motor inrush currents, are different from arc currents in one very important respect; inrush currents are much more regular and predictable. Exemplary types of inrush currents are described below.
The capacitive inrush current is exponential in shape, shown in FIG. 10, with a time constant determined by circuit resistance and load capacitance, e.g., an electronic device that has a capacitor bank on the input, such as a flight computer, data acquisition box (“black box”) or similar piece of avionics equipment. Lamp inrush currents occur when there is a change in resistance from a cold filament to a hot filament in an incandescent lamp. An exemplary waveform for a lamp inrush current is shown in FIG. 11. However, lamp inrush currents vary with lamp type. A motor inrush current occurs when a motor starts.
An effective arc current detector will ignore all of these types of inrush currents and only trip when an actual arc is detected. Therefore, there is a need for an improved arc detection circuit.